Vocal characteristics of pygmy blue and their change over time

Alexander N. Gavrilov,a) Robert D. McCauley, and Chandra Salgado-Kent Centre for Marine Science and Technology, Curtin University of Technology, G.P.O. Box U1987, Perth, Western Australia 6845, Australia

Joy Tripovich School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, New South Wales 2052, Australia

Chris Burton Western Research, P.O. Box 1076, Dunsborough, Western Australia 6281, Australia (Received 1 June 2011; revised 7 September 2011; accepted 13 September 2011) Vocal characteristics of pygmy blue whales of the eastern population were analyzed using data from a hydroacoustic station deployed off Cape Leeuwin in Western Australia as part of the Comprehensive Nuclear-Test-Ban Treaty monitoring network, from two acoustic observatories of the Australian Integrated Marine Observing System, and from individual sea noise loggers deployed in the Perth Canyon. These data have been collected from 2002 to 2010, inclusively. It is shown that the themes of pygmy songs consist of ether three or two repeating tonal sounds with harmonics. The most intense sound of the tonal theme was estimated to correspond to a source level of 179 6 2dBre1lPa at 1 m measured for 120 calls from seven different animals. Short-duration calls of impulsive downswept sound from pygmy blue whales were weaker with the source level estimated to vary between 168 to 176 dB. A gradual decrease in the call frequency with a mean rate estimated to be 0.35 6 0.3 Hz/year was observed over nine years in the frequency of the third harmonic of tonal sound 2 in the whale song theme, which corresponds to a negative trend of about 0.12 Hz/year in the call fundamental frequency. VC 2011 Acoustical Society of America. [DOI: 10.1121/1.3651817] PACS number(s): 43.30.Sf, 43.80.Ka, 43.30.Nb [WWA] Pages: 3651–3660

I. INTRODUCTION distinct from those of the other pygmy blue whales in the In- dian Ocean (Alling et al., 1991). This population has been Pygmy blue whales are a subspecies of blue whales suggested to comprise of a separate subspecies referred to as (Balaenoptera musculus) inhabiting the Indian Ocean and Balaenoptera musculus indica. the South-west Pacific (Rice, 1998). The species found in the Characteristics of pygmy blue whale vocalization have southwestern part of the Indian Ocean south off Madagascar been considered in several publications. McDonald et al. and in the eastern Indian Ocean west off Australia and Indo- (2006) summarized data from passive underwater acoustic nesia is commonly referred to in the biological literature as observations of whales distinguishing blue whale subspecies Balaenoptera musculus brevicauda (Ichihara, 1966). Pygmy and populations by the of their songs. Stafford blue whales have also been observed in the Southern Ocean et al.(2010)reported different call structures of pygmy blue from the Great Australian Bight to Bass Strait (Gill et al., whales from the Sri Lanka, Madagascar and Australian popu- 2011). They are believed to belong most likely to the same lations in the Indian Ocean. McCauley et al. (2001) analyzed population as pygmy blue whales in the eastern Indian the structure of pygmy blue whale songs of the eastern Indian Ocean (Branch et al., 2007). This is proven to a certain Ocean population observed in Western Australia. Samaran extent by the same song structure of whales observed in the et al. (2010) measured the source level of calls from pygmy Indian and Southern Oceans in this study. The whales from blue whales of the Madagascar population, using acoustic the southwestern Pacific population found mainly north off data from the comprehensive nuclear-test-ban treaty (CTBT) New Zealand and around the Southwest Pacific Islands differ hydroacoustic station off Crozet Island in the Southeast Indian from the pygmy blue whales of the eastern Indian Ocean Ocean. Assuming a simple spherical spreading model for the population in their size and in the sounds they produce transmission loss and using hyperbolic location of whales (McDonald, 2006). A separate population of pygmy blue from a triangular array of the hydroacoustic station, the source whales was identified in the Northern Indian Ocean. The ani- level of whale calls was estimated to be 174 6 1dBre1lPa mals from this population appear to stay year-round within a at 1 m, which is at least 10-dB lower than the estimates made limited area between Somalia and Sri Lanka and make calls for other blue whale subspecies (e.g., Thode et al., 2000, McDonald et al.,2001and Sˇirovia´c et al.,2007). a)Author to whom correspondence should be addressed. Electronic mail: Recently, McDonald et al. (2009) made an interesting [email protected] discovery revealing a gradual decrease of tonal frequencies

J. Acoust. Soc. Am. 130 (6), December 2011 0001-4966/2011/130(6)/3651/10/$30.00 VC 2011 Acoustical Society of America 3651

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp in songs of different blue whale subspecies worldwide. This observation was unambiguous only for the northeastern Pa- cific population of blue whales, where the number of calls and animals sampled was large enough to estimate the mean call frequency and the duration of observations was long enough to reveal an interannual trend. However, for pygmy blue whales in the Indian Ocean, the number of animals and samples made in different years was too small to make a reli- able conclusion regarding trends in the call frequency. Although the population of pygmy blue whales is believed to have been growing since the end of commercial whaling, little is published about the abundance and migra- tion patterns of pygmy blue whales in the eastern Indian and Southern Oceans. McCauley and Jenner (2010) reported on the northward and southward migratory phases of pygmy blue whales travelling along the Western Australia coast. Based on passive acoustic detections, they estimated the number of whales, migrating southward past Exmouth in 2004, to be between seven and fifteen hundred. For monitor- ing the population and studying the migration of pygmy blue FIG. 1. Locations of the HA01 CTBT station (black triangle) and two whales by means of passive acoustic observations in the IMOS acoustic observatories (black squares) used for studying vocalization ocean, the structure of whale songs and acoustic characteris- characteristics of pygmy blue whales. tics of individual calls need to be known. For example, shown in Fig. 1. Three acoustic receivers of the HA01 station knowing the song structure is crucial for determining the are moored at a depth of about 1100 m below the sea surface number of vocalizing whales within a certain time period. at approximately 2 km from each other to form a triangular The acoustic source level of whale vocalization is required array. The timing accuracy of signal sampling in all three for estimating the detection range in the ocean from a pas- receive channels is 1 ms, provided by synchronization to GPS sive acoustic station. The structure and frequency of whale clock. The are calibrated to ensure the accuracy calls and their changes over time need to be known to design of acoustic pressure measurements of 61 dB within the fre- an efficient acoustic detector of vocalizing whales. quency band of 10–100 Hz. The seafloor around the station is The aim of the study presented in this article was to ana- gently sloping with the sea depth of about 1600 m at the array lyze the structure and acoustic characteristics of vocaliza- center. The receiving system records sea noise continuously tions produced by pygmy blue whales of the eastern Indian at a sampling frequency of 250 Hz and communicate acoustic Ocean population and to examine changes in these character- data to the shore in real time via an underwater cable. istics over time. This analysis was made using (1) acoustic The IMOS acoustic observatories consist of four autono- data collected at the CTBT hydroacoustic station off Cape mous sea noise loggers set on the seafloor as a triangular Leeuwin in Western Australia (referred to as HA01 in the array of about 5-km sides with the fourth logger placed at CTBT nomenclature) in 2002 – 2007, (2) underwater acous- the array center. Sea noise recordings are programmed to tic recordings made by the passive acoustic observatories of sample 500 s starting every 900 s. The sampling frequency is the Integrated Marine Observing System (IMOS) deployed 6 kHz and the upper limit of the frequency band is 2.8 kHz at in the Perth Canyon in Western Australia and off Portland in 3 dB. Sea noise is recorded almost year round with a short Victoria in 2009 – 2010, and (3) sea noise data collected in interruption for system redeployment with data retrieval ev- the Perth Canyon before 2009 under support from Australian ery eight to twelve months. Defense. Recordings of sea noise and whale calls in the Perth Can- The passive acoustic observing systems and the data used yon in 2005 and 2008 were made on a single bottom-mounted for the analysis presented in this article are described in Sec. sea noise logger. Continuous recordings of 200 s long were II. The structure of pygmy blue whale songs and their individ- made at a sampling frequency of 6 kHz and repeated every ual calls are discussed in Sec. III. The source level of whale 900 s over approximately six months in both seasons. vocalization is analyzed in Sec. IV using hyperbolic location of signals sources from a triangle array of the HA01 station III. WHALE SONG STUCTURE and numerical modeling of acoustic propagation at the station. The location errors and ambiguity are also considered. Long- Songs produced by pygmy blue whales of the eastern In- term changes in acoustic characteristics of whale calls dian Ocean population consist of a series of sounds, some- observed from 2002 to 2010 are discussed in Sec. V. times referred to as themes, repeated with more or less regular intervals over a long time from several tens of minutes to hours. The most common theme observed in Perth Canyon II. DATA COLLECTION consists of three quasi-tonal multi-harmonic sounds made The locations of three passive acoustic stations used for within approximately 120 s from the theme initiation to the detection of pygmy blue whales and recording their calls are end of the last sound (top panel in Fig. 2.). The introductory

3652 J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp of lower spectral levels. A series of tones offset from the har- monics of the fundamental frequency can also be distin- guished in this sound, when the signal-to-noise ratio is high, which may indicate that there is another source of sound in the whale vocal apparatus. The most typical repetition interval of the whale songs consisting of the three-sound theme is about 190 6 5s.How- ever, it can vary between different songs and animals from approximately 175 s to 220 s. There are probably other factors of environmental or behavioral origin that affect the theme repetition rate of individual animals. Whale songs consisting of the three-sound theme were also often recorded at the HA01 station off Cape Leeuwin and at other locations along the continental slope of Western Australia. However, more than half of the songs detected off Cape Leeuwin have the in- troductory sound omitted in the periodical themes, while the second and third sounds of the theme preserve similar fre- quency and time characteristics to those of the three-sound theme. Moreover, when the whale skips the introductory sound, the theme repetition interval reduces to approximately 80 s (bottom panel in Fig. 2). There is no obvious explanation why pygmy blue whales alter their song theme and the theme repetition interval. The difference in the whale behavior in different areas can be one of the likely reasons for such an alteration. It is believed that the area around Perth Canyon canbeafeedinggroundforpygmybluewhales(Rennie et al., 2009), so whales may stay there for many weeks, if the productivity is high within a season. In contrast to the Perth FIG. 2. Spectrograms of pygmy blue whale songs consisting of repeating Canyon, the continental shelf and slope area southwest of three-sound themes (top panel) and two-sound themes (bottom panel). Spec- Cape Leeuwin is thought to be a region the whales travel trogram parameters: 1024-point FFT, 1024-point Hanning window with 95% overlap. through during migration, so most of the whales recorded are believed to be moving in a certain direction. One of the authors of this article made simultaneous vis- sound (or sound 1 of the theme) is usually the weakest and ual and acoustical observations of a pygmy blue whale in longest one lasting up to 50 s. The principal frequency of this Geographe Bay in Western Australia and recorded calls from call is about 20 Hz. It often increases stepwise by about 1 Hz this animal that were totally different from the tonal sounds in the middle of calls. The power spectrum of this call spans commonly produced by pygmy blue whales. These calls were frequencies typically to 70–80 Hz. This introductory sound is impulsive signals of about 1 to 2 s long with the call fre- least stable compared to the other two sounds in terms of the quency changing rapidly with time (top panel in Fig. 3). Simi- amplitude, duration, frequency content and the consistency it lar calls were also found in the acoustic recordings made off appears in the song theme. Cape Leeuwin (bottom panel in Fig. 3) and in the Perth Can- The introductory sound, when it is present, is followed yon during the presence of pygmy blue whales in the listening by the second sound (sound 2) after a pause of a few sec- areas. Based on these observations, this type of call was attrib- onds. This sound is usually the most intense and invariable uted to pygmy blue whales. The longest and most intense part one. The fundamental frequency of this call always increases of the call is a downsweep signal with the principal frequency from 20–21 Hz to 23–24 Hz during the call duration of about decreasing rapidly from 70–100 Hz to 20–50 Hz within differ- 25 s, so do the multiple harmonics. The third harmonic is ent frequency bounds in different calls. A similar type of blue most prominent after the principal frequency in the signal whale call, sometimes referred to as a D-type call, is fre- spectrum. In a noisy ocean environment, when the noise quently observed in the Northern Pacific (Oleson et al.,2007) level decreases noticeably with the frequency increase from and in the Southern Ocean (Rankin et al.,2005). In Australia, 20 Hz to 70 Hz, the third harmonic of this call often has the this type of pygmy blue whale call is less common than the maximum signal-to-noise ratio (SNR) and hence can be used tonal sounds. In contrast to the song themes of tonal sounds, as the main feature for call detection. the D-type calls are not repeated with regular intervals. The last sound of the theme usually starts about 20 s af- ter the end of the previous sound and lasts for approximately IV. SOURCE LEVEL OF PYGMY BLUE WHALE CALLS 20 s. Its spectrum consists of many spectral lines that do not change frequency noticeably during the call. These spectral To estimate the source level of whale calls, it is necessary lines include the principle frequency of 19–20 Hz, its har- to (1) detect calls from a vocalizing whale; (2) accurately monics of high amplitude and a number of other frequencies measure the acoustic pressure of the received signal; (3)

J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization 3653

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp FIG. 3. Spectrograms of downsweep signals of pygmy blue whales calls recorded in Geographe Bay (top panel) and off Cape Leeuwin (bottom panel) in Western Australia. parameters: 256-point FFT, 128- point Hanning window with 95% overlap. measure the distance to the calling whale; (4) have some knowledge of the most likely source depth; (5) estimate the transmission loss (TL) at the measured distance and expected source depth using either empirical or numerical acoustic propagation models; and (6) estimate the source level reduced to a reference distance, which is usually 1 m. Whale detection can be implemented either through visual inspection of sea noise spectrograms or via an automatic searching algorithm capable of recognizing calls by certain whale species in sea noise. Measuring the received level is straightforward if the acoustic receive system is accurately calibrated. The accuracy FIG. 4. Located positions of two vocalizing whales (small dots) making of the other three operations greatly depends on the measure- regular tonal calls (top panel) and sporadic downsweep calls (bottom panel) ment geometry and the knowledge of the environment. recorded in 2002 and 2003 respectively; the relative location of the HA01 The triangular array of the HA01 station was used to hydrophones (black circles); location ambiguity zones (shadowed areas in the top panel); and error ellipses of location at 95% confidence level (dashed locate vocalizing whales and estimate the source level of ellipses). The arrow in the top panel shows the general travel direction of their calls. The array geometry is shown in Fig. 4. the singing whale with the mean speedhi V .

A. Whale detection and measurements of received from other sources with similar spectral characteristics, e.g., signal level shipping noise. Only the signals of a high SNR were chosen A signal recognition algorithm for automatic detection of from several thousands of detected calls for locating whales. pygmy blue whale calls was designed and tested using the For each event of successful location of a vocalizing whale, CTBT and IMOS passive acoustic data. The algorithm the received signal level was measured for sound 2 of the searches for transient signals in sea noise with time-frequency song theme, as the most intense one, at all three hydrophones features similar to those of the first and third harmonics of of the receive array. The level was estimated from the mean theme sound 2, which is common for all whale songs. These intensity of the signal band-pass filtered in 22–25 Hz and features are the signal duration, the frequency band and the 66–75 Hz frequency bands, which spanned the principal fre- slope of frequency change with time. The algorithm demon- quency and its third harmonic containing more than 90% of strated misdetection and false detection rates of less than 5%. the signal energy. The measured signal intensity was also cor- Misdetected calls were either very weak or hidden in noise rected for the noise intensity, i.e., squared RMS amplitude,

3654 J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp estimated in the same frequency bands. Signals with the SNR (2) It is difficult to allow for the effect of multipath propaga- of less than 6 dB were not considered in further analysis of tion in TDOA estimates by locating maxima of the signal thesourcelevel. cross-correlation. Cross-correlating signal spectrograms The received level of downsweep signals from pygmy rather than waveforms reduces the multipath effect to a blue whales was estimated from the maximum RMS ampli- certain extent (Samaran et al., 2010), but at the cost of tude of acoustic pressure measured in a 0.5-s window in a considerable degradation of the TDOA measurement ac- broad frequency band of the signal spectrum. The measured curacy. The problem can be partly conquered by ignor- signal levels were corrected for the mean noise level imme- ing events of inconsistent TDOA measurements when diately before the signal detection. Dt12 6¼ Dt13 Dt23; Dtij is the TDOA measurement for receivers i and j. B. Location (3) Because the TDOA measurements contain errors due to Location of an underwater acoustic source using a trian- limited SNR and multipath propagation effects and the gular array, similar to that of the HA01 CTBT hydroacoustic actual positions (X, Y, Z) of the receivers and the source station, is not a trivial problem. Firstly, at least five receivers depth are known only to a certain accuracy, the location are needed to unambiguously locate a point source in three- problem can be solved only in the least squares (LS) dimensional space using a hyperbolic method (Spiesberger, sense. The functional to be minimized is 2001), if the time differences of signal arrivals (TDOA) are accurately measured and the sound speed is known. Secondly, X W X ; Y Dt R R =C 2; (1) the ocean acoustic environment is not free space. It is ðÞ¼S S ij i j i;j6¼i bounded by the sea surface and seafloor and commonly has depth dependent sound speed, which causes multipath propa- where R ¼½ðÞX X 2þðÞY Y 2þðÞZ Z 21=2 is gation from an acoustic source to a receiver. Acoustic signals i S i S i S R the distance from the source to receiver i located at X , propagated along different paths interfere with each other, i Y and Z , Z isthesourcedepth,X and Y are horizon- resulting in errors of the TDOA measurements for direct sig- i R S S S tal coordinates of the source to be estimated, and C is nal arrivals using cross-correlation of signals at different the sound speed. Here we assume that the sound speed hydrophones. Finally, the acoustic environment in the obser- averaged along direct paths to all receivers is the same. vation area, including the sound speed profile, bathymetry We can also assume that errors in the distances R due and acoustic properties of the seafloor, is not absolutely i to uncertainties in the receivers’ positions X , Y and Z known in practice. To overcome these problems, one can i i R and the source depth Z can be allowed for in the errors make the assumptions and implement the approaches as C of TDOA measurements. Moreover, in the LS approach follow: to the hyperbolic location problem, the requirement for (1) The receivers’ depth is usually known to certain accu- the TDOA measurements to be consistent for three racy (about 1100 m for the HA01 hydrophones). The pairs of receivers can be less strict: depth of the source to be located (e.g., whale) is usually unknown. However, some assumptions with respect to jjDt12 Dt13 Dt23 < dt; the most likely source depth and its possible variations can be made based on available data. According to the where dt is the criterion of the TDOA discrepancy measurement made by Oleson et al. (2007), blue whales chosen to omit measurements resulting in large loca- of the northeast Pacific population make quasi-tonal tion errors. calls, somewhat similar to those of Australian pygmy A Levenberg-Marquardt method (Bjo¨rck, 1996) for solv- blue whales, at depths varying from 20 to 30 m, i.e., near ing non-linear LS problems was used in this study to locate the sea surface in a deep-water environment. If the sound pygmy blue whales from the triangular array of the HA01 sta- production physiology is similar for blue whales regard- tion. Sound 2 of the song theme was chosen for TDOA meas- less of their habitation area, then we can assume that urements, as it was generally the most intense sound of pygmy blue whales make tonal calls within the same pygmy blue whale songs. The TDOA maximum discrepancy depth range. Once the receivers’ and source depths are dt was chosen to be 0.05 s, which is about one period of the determined, the 3D hyperbolic location problem reduces principal frequency of this sound. The detected whales were to a 2D scenario. However, a triangular array of three located in two iterations. Firstly, possible locations of a vocal- receivers has zones of ambiguous location (shadowed izing whale were searched for by using a set of initial guesses zones shown in the top panel of Fig. 4), where two solu- distributed on an equidistant grid spanning an area of 10 by tions of the hyperbolic equation system exist for the 10 km wide around the receive array. The grid size was same set of TDOA data. If the location solution falls into 0.5 km. The LS algorithm stopped searching the solution ei- one of those zones, then such a location is either ignored ther after 1000 iterations or when two consecutive iterations 2 2 in further analysis of the source level or compared to the did not change the criterion v ¼ W=rt by more than 0.01, 2 2 other possible solution to select the most likely one where rt ¼ dt was the initial guess for the variance of the based on other criteria, e.g., the locations of previous TDOA measurements common for all three pairs of receivers. calls by a periodically vocalizing whale and the feasible The solution of smallest covariance (smallest error ellipse) speed of its motion. was taken as a coarse location estimate. If two solutions with

J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization 3655

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp significantly different locations and small covariance were floor at steep angles. The sound field was modeled at frequen- found, then one of those was selected based on the previous cies from 20 Hz to 75 Hz with a 0.5-Hz increment for two locations of the vocalizing whale. The selected location was sources depths of 20 m and 30 m. The sound speed profile was used as an initial guess for refining location and estimating calculated from the World Ocean Atlas 2009 seasonal temper- location errors in the second iteration, where the variance ature and salinity data. Acoustic properties of the seafloor 2 rt of TDOA measurements was updated separately for each were modeled based on results of geoacoustic inversion for pair of receivers, based on the coarse location estimate and airgun signals recorded at the HA01 station at different dis- the variance of the receivers’ horizontal positions and source tance from 20 km to 80 km from the signal source (Gavrilov depth. The standard deviation of the HA01 hydrophones from et al., 2007). A simple fluid half-space model was assumed their relative horizontal position in the triangular array was with the geoacoustics properties corresponding to medium to estimated to be most likely less than 20 m (Li and Gavrilov, coarse sand: compressional wave velocity of 1800 m/s, den- 2007). The source depth was assumed to be 25 6 5m. sity of 2100 kg/m, and sound attenuation of 0.15 dB/m kHz. Locations of a whale traveling through the HA01 array The transmission losses averaged in the two frequency bands and making regular tonal calls are shown in the top panel in of signal filtering are shown in Fig. 7. The effect of source Fig. 4. The whale was moving generally in the north- depth is noticeable at distances less than approximately 3 km. northwestern direction, although its track was not rectilinear. Beyond this transition range, the transmission losses tend to The average travel speed was 5 to 6 km/h. Most of the calls follow a cylindrical spreading law and their changes are not by this whale were located outside the ambiguity zones. The significant when the source depth varies from 20 m to 30 m. error ellipses of 95% confidence level shown in this figure For lack of adequate environmental data, especially decreased rapidly as the animal approached the array. those related to the acoustic properties of the bottom, some Another six whales traveled close to the HA01 station in authors (e.g., McDonald et al., 2001 and Samaran et al., 2002 – 2006, such that their locations during vocalization 2010) used a simple spherical spreading model to predict the could be determined more or less accurately. Some of these transmission losses. This could be a reasonable approxima- whales were also travelling in the north-northwestern direc- tion at relatively short distance, if two factors were taken tion. The distance from each located position of vocalizing into account. Firstly, for a shallow source and deep receivers whales to all three receivers of the HA01 station was used to in deep water, the slant distance between the source and the estimate the source level of whale calls. receiver must be used rather than the horizontal range. Sec- A series of downsweep calls of type D from pygmy blue ondly, the signal from a shallow source reflected from the whales that were close and strong enough to be located from sea surface contributes to the received signal energy nearly the HA01 station was recorded in early March 2003. These as much as the direct signal. Consequently, the transmission calls were definitely from a few different animals, although losses are expected to be more accurately approximated with the actual number of vocalizing whales could not be identi- the following range dependence: fied. The whales did not cross over the receive array while hi vocalizing and stayed farther from the array than the whales 1=2 TL 20 lg R2 þ Z2 3dB; (2) located by their tonal sounds. The SNR of these calls was R much lower than that of the tonal calls used for location. Nevertheless, the TDOA measurements were accurate where R is the horizontal distance to the source and ZR is the enough to locate several calls, which was due to a much receiver depth. The source depth is neglected in Eq. (2),as broader frequency band of the downsweep signals. The loca- small compared to ZR. The transmission loss calculated from tions of the calling whales detected from the HA01 station is Eq. (2) and shown in Fig. 5 by the dash-and-dot line approxi- shown in the bottom panel of Fig. 4. The approximate detec- mates reasonably well the numerically predicted transmis- tion time of whale vocalizations is also shown at each loca- sion losses up to the transient range of about 3 km, beyond tion. Based on the locations and times of four calls detected which the numerically predicted and approximate TL curves between 7:26 and 7:36, one can conclude that one animal gradually diverge tending to the spherical and cylindrical was moving in the north-northeastern direction away from decay rates, respectively. the acoustic array. The other six calls are difficult to interpret The received levels of 120 signals from seven pygmy with respect to whale identification and motion. blue whales located close to the HA01 station in 2002–2006 are shown in Fig. 6. The measured signal level decreases C. Source level of whale calls almost consistently with range with relatively small disper- sion. Moreover, there is no evident difference between the 1. Tonal calls levels of signals received from different animals at the same Transmission losses (TL) were estimated using both nu- ranges. The sound from the whale observed in 2006 was merical modeling and a simple approximate equation. slightly more intense at shorter distances than that from the Because (1) the signal source was located at relatively short other whales, but only by 1–2 dB. Using the numerically mod- distances compared to the sea depth of about 1600 m and (2) eled TL curves for estimating the source level was not the seafloor within the area of whale location was rather flat, straightforward, because the source depth, that greatly affects the numerical prediction of TL was made using a wavenum- the signal level at short distances, was not precisely known. ber integration method (Jensen et al., 2000). This allowed us For this reason, we used the spherical spreading approxima- to take into account the signal energy reflected from the sea- tion given by Eq. (2) to estimate the source level. The best fit

3656 J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp FIG. 5. Acoustic transmission losses averaged in two frequency bands of 22–25 Hz and 66–75 Hz, modeled for the ocean acoustic environment at the FIG. 7. Received signal levels measured for ten downsweep signals HA01 station and two source depths of 20 m (dashed line) and 30 m (solid from pygmy blue whales received at different distances from the HA01 line). The dash-and-dot line shows the approximate TL curve based on hydrophones. spherical spreading of acoustic energy with slant range to the source near the sea surface and corrected for the contribution from the surface reflected signal. V. LONG-TERM CHANGES IN PYGMY BLUE WHALE SOUND of the experimental measurements by the approximate TL McDonald et al. (2009) found recently that the tonal fre- curve is shown in Fig. 6 by the dashed line. It corresponds to quencies of blue whale songs had been gradually decreasing the source level of 179 dB with 2-dB standard deviation. in several parts of the world ocean since the earliest record- ings in the mid 1960 s. This astonishing finding was primarily 2. Downsweep calls supported by a long series of observations made in the north- The signal levels of downsweep calls from pygmy blue eastern Pacific from 1963 to 2008. The number of different whales measured at the HA01 hydrophones are shown in animals assessed with respect to their call frequency varied Fig. 7. The first observation from this plot is that the from one to ten in different years of the observation period. received signal level varied among different calls much These authors also noticed that pygmy blue whales of the more than with distance to the calling whale. At distances eastern Indian Ocean population had slightly higher call fre- from 3 km to 5 km from the calling whales, the received sig- quencies in 1993 than those recorded in 2000. The compari- nal levels varied from about 101 dB to 109 dB re 1 lPa. This son was made for six calls of two different animals recorded means that the source level of different downsweep calls by in 1993 and about 200 calls from an unknown, but presum- pygmy blue whales varies considerably in contrast to the ably small number of whales recorded in 2000. A similar ob- tonal calls. Based on the approximate TL curve given in Eq. servation was made in this study based on the acoustic (2) and the signal levels measured at 3–5 km, the source recordings at the HA01 station. Figure 8 shows a superposi- level of downsweep calls might vary from about 168 dB to tion of two spectrograms of sounds 2 and 3 in pygmy blue nearly 176 dB re 1 lPa at 1 m. whale calls recorded in March 2002 and 2007. The structure of the whale call recorded in 2007 was similar to that in

FIG. 6. Received signal levels measured for seven whales located at differ- ent distances from the HA01 hydrophones. The dashed line shows the best FIG. 8. Superposition of two spectrograms of theme sounds 2 and 3 fit of the measured level by the approximate TL curve given in Eq. (2). recorded from different whales in 2002 and 2007.

J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization 3657

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp 2002, but the principal frequencies and their harmonics were effect of variations between different animals, the mean value slightly lower. The frequency of the third harmonic in sound of the maximum frequency was calculated for all calls 2 and the higher harmonics of sound 3 decreased by about recorded in each two-week period, if the number of calls was 1.5 Hz. To obtain more accurate estimates of the frequency not less than ten. The standard deviation of frequency varia- change, we measured the maximum frequency of the third tions was also calculated. Because the CTBT acoustic data harmonic in all calls recorded from these two different from the HA01 station were available only until the beginning whales. This frequency was 72.9 6 0.2 Hz in 2002 and of 2008, we extended the time series using acoustic record- 71.2 6 0.2 Hz in 2007 measured from 25 calls of each ani- ings made at the IMOS acoustic observatory in the Perth Can- mal. However, comparing calls from a small number of ani- yon in 2008 – 2010, where many thousands of calls were mals is not sufficient for making conclusions regarding recorded every season. Moreover, to make sure that the region general trends in vocalization characteristics of the entire of whale observation did not significantly affect the call fre- whale population. The calls compared could be from whales quency, we also analyzed the whale calls recorded in Perth of different age and size and, consequently, the frequency of Canyon in 2005 and at the IMOS acoustic observatory off their calls could be expected to differ to a certain extent. Portland in Victoria in 2009. From several hundreds to thousands of pygmy blue whale The top panel in Fig. 9 shows the number of calls calls were recorded every summer-autumn season at the detected fortnightly at the three observational sites and used HA01 hydroacoustic station and the Perth Canyon from 2002 for measuring the call frequency. The number of calls to 2010. These data represent hundreds of whales migrating recorded in the Perth Canyon is an order greater than that at seasonally along the coastal shelf and continental slope off the other two sites, likely because the Perth Canyon is a rela- Australia’s South West and, therefore, can be used to assess tively small geographical area attracting many pygmy blue long-term changes in the vocal characteristics of pygmy blue whales as a regular feeding ground. The bottom panel of Fig. whales of the eastern Indian Ocean population. Sound 2 of the 9 shows the mean value of the maximum frequency meas- typical song theme was chosen to analyze the frequency ured at the third harmonic of sound 2 in the whale song change, as the most persistent and best-detected sound in theme. The error bars in the legend indicate typical standard pygmy blue whale songs. The frequency of the third harmonic deviation of the frequency variations observed every two was measured, as it was most indicative of absolute change in weeks. It is important to note that these variations could be the call frequency. Because the frequency of this whale sound due not only to the vocal variability among individual increases slightly with time during the call, it is necessary to whales, but also to selective-frequency fading, which results choose a reference point in either the temporal or frequency from multipath acoustic propagation and affects the signal domain for measuring frequency variations between different power spectrum including its maximum frequency, depend- calls. We measured the maximum frequency of the third har- ing on range and depth of the signal source. monic at the end of whale calls, which was determined from A downward trend in the interannual changes of the call the power spectrum of this harmonic by a 3-dB decrease in frequency is evident from this observation. The total decrease the spectral level towards higher frequencies. To reduce the over these years was much larger than the variations during

FIG. 9. The number of whale calls detected fortnightly at the HA01 station of Cape Leeuwin, in Perth Canyon and off Portland that were used to measure the mean value and standard deviation of call frequency (top panel) and the mean value of the maximum frequency of the third harmonic in sound 2 of the whale song theme measured at these three sites in 2002–2010 (bottom panel). Asterisks rep- resent data from the Cape Leeuwin sta- tion, circles, the Perth Canyon, and pluses, the Portland IMOS observatory.

3658 J. Acoust. Soc. Am., Vol. 130, No. 6, December 2011 Gavrilov et al.: Pygmy blue whale vocalization

Downloaded 15 Dec 2011 to 134.7.248.132. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp particular seasons. A regression analysis applied to the tempo- occurrence, duration and spectral characteristics. Varying ral variations of the fortnightly averaged call frequency from 168 dB to 176 dB re 1 lPa at 1 m, the source level of showed a decrease rate of approximately 0.35 Hz/year with these calls is noticeably lower and much less stable than that the standard error of about 0.01 Hz/year (R2 statistics value of of the tonal sounds of blue whale songs. approximately 0.95 and t-statistics p-value less than 0.001 at The only significant change in the acoustic characteris- 101 degrees of freedom). A more conservative estimate of tics of vocalizing pygmy blue whales observed over eight possible errors based on the maximum standard deviation of years in Australia was a noticeable decrease of the tonal fre- about 0.8 Hz for all fortnightly measurement sets resulted in a quencies in whale songs. The third harmonic of sound 2 in 95% confidence interval of approximately 0.35 6 0.06 Hz/ the song theme has decreased by approximately 3 Hz over year for the decrease rate. Thus the trend estimation errors these years, which corresponds to a 1-Hz decrease of the were considerably smaller than the measured interannual principal frequency. Moreover, a comparison of individual trend. The decrease rate of 0.35 Hz/year estimated for the calls recorded in different years showed that the frequencies third harmonic of sound 2 corresponds to an approximately of the third sound in the typical song theme have also likely 0.12 Hz/year decrease rate of the fundamental frequency and decreased in a similar way. This observation strongly sup- is close to a 0.4 Hz/year decrease rate estimated by McDonald ports the conclusion made by McDonald et al. (2009) regard- et al. (2009) for the third harmonic (from about 65 Hz to ing a worldwide decline in tonal frequencies of blue whale 45 Hz) of the tonal sounds produced by blue whales of the songs. These authors also considered a number of possible northeastern Pacific population. A downward trend in the reasons for such change, which included: ; mean value of the call frequency can be seen even during one an increase in body size due to, in particular, the population year of data collection in the Perth Canyon in 2010, although recovery from whaling; global climate change and ocean the observed change is noticeably smaller than the standard acidification resulting in sound speed change; biological in- deviation of variations. terference with other marine ; and an impact of man-made noise. None of these reasons look plausible, except an increase in the relative population density and a VI. DISCUSSION consequent slight decrease of the call intensity suggested by The structure and acoustic characteristics of songs and the authors as the only possible but yet speculative explana- individual calls from pygmy blue whales of the eastern Indian tion for the frequency decline of blue whale songs. The most Ocean population were considered in this paper. The long- significant change of blue whale source level from 188.4 dB lasting songs are formed by repeating themes of either three in 1963 to 185.3 dB in 2008 was estimated for whales of the or two different quasi-tonal sounds with harmonics spanning northeastern Pacific population. The study presented in this the frequency band from about 20 Hz to 100 Hz. The theme paper did not reveal any noticeable change in the source repetition period in a three-sound song varies from 180 s to level of pygmy blue whale calls over eight years of data col- 220 s. The first sound is skipped in a two-sound song, which lection. This should be expected, because errors of source has the theme repetition period of 90–100 s. These two songs level measurements for whales are certainly larger structures and their individual sounds remained generally than 6 1 dB, which includes errors of location and TL pre- unchanged over eight years of passive acoustic observations diction. Moreover, the intensity of calls can vary among ani- off the western and southern cost of Australia. mals during one season, depending on their size, age and The second sound in the three-sound theme (first in the behavioral context. Therefore, to assess general trends in the two-sound theme) is usually the most intense one. The source intensity of blue whale calls, the source level of many ani- level of 179 6 2 dB estimated for this sound from 120 calls mals should be accurately measured over a much longer did not vary significantly between seven different whales time period. The probability of long-term trends can be recorded at the HA01 hydroacoustic station over 2002–2006. determined only if short-term variations observed within dif- This estimate of the source level is noticeably higher than ferent years are taken into account. the estimate of 174 6 1 dB reported for pygmy blue whales There is another hypothesis regarding the decline in call of the southwestern Indian Ocean population observed in frequency with time that was overlooked in McDonald at al. 2003–2004 (Samaran et al., 2010). It is not certain yet (2009). The resonance frequency of an air-filled elastic whether this difference in the source level is due to possible body, e.g., the respiratory system of whales, changes with distinctions in average size or vocalization habit of these two depth in water. According to Jones et al. (2002), this change 5=6 populations or the estimate made in the western Indian Ocean can be approximated as f1=f2 ½ðÞ1 þ d1=10 =ðÞ1 þ d1=10 was not representative or accurate enough. The source level and f1=f2 ½ðÞ1 þ d1=10 =ðÞ1 þ d1=10 for spherical and estimate of 174 dB was obtained only for one animal, using a cylindrical bodies respectively, where f1 and f2 are resonance 2D model for hyperbolic location, which ignored the depth frequencies at depths d1 and d2 respectively. If the resonance difference of the source and receivers, and assuming a simple frequency was 24 Hz at 25 m, then it is expected to drop to spherical spreading model for the transmission loss. 23 Hz at 23.3–23.5 m, depending on body shape. This small Like some other sub-species of blue whales, pygmy change in the average depth of blue whale vocalization looks blue whales of the eastern Indian Ocean population produce quite realistic in explaining the observed decline of call impulsive downsweep sounds of 2–4 s long within the fre- frequency; however, it is not possible at present to justify this quency range from about 20 Hz to more than 100 Hz. This hypothesis and provide a reasonable explanation as to why it type of calls is relatively infrequent and irregular in terms of may be occurring.

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